AIDS, Zinc Deficiency, and Thymic Hormone Failure

To the Editor. —It has been proposed that thymic secretory function, as assessed by determining levels of circulating thymulin—like activity,¹ is greatly reduced in patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex.^2,3 In fact, in both young³ and adult² patients, levels of circulating thymulin, as measured by the rosette inhibition bioassay, are either undetectable or below the values found in age-matched healthy subjects. These findings have been interpreted as showing human immunodeficiency virus (HIV)—induced injury of the thymic epithelial cells that synthesize and release thymic hormones.⁴

Since thymulin is biologically active, and therefore measurable by bioassay only when the nonapeptide thymulin is bound to a zinc ion,⁵ a reduced circulating level may depend not only on defective production by the thymus, but also on decreased zinc binding, as might occur in zinc deficiencies.⁶ In these conditions, inactive, zinc—unbound thymulin molecules are found